Graphene has attracted much attention since its discovery in 2004. Graphene is a one atom thick material composed of carbon atoms structured in a honeycomb hexagonal lattice. To put this in context we have to mention that the thickness of graphene is 0.345 nm, one million times thinner than an A4 piece of paper. Graphene's exotic properties such as high electronic mobility, extraordinary thermal conductivity, great strength, flexibility, and transparency make it an ideal candidate in many different applications. Graphene could have applications in electronics (high frequency devices, transistors, etc.), in energy (solar cells, batteries, supercapacitors, etc.), in touch screen and display technology (TV screens, mobile phones, etc.), in sensors and many more. As a consequence, the interest in graphene has increased exponentially in number of academic publications and patent applications.
Most of the applications foreseen for graphene will require a large-scale production of this material. At present, graphene can be manufactured using a variety of techniques and depending on the method the quality of the graphene obtained is very different. Graphene fabrication methods can be classified into two groups: the top down and the bottom up approach.
In the top down approach, graphene is prepared starting from graphite via the chemical or mechanical exfoliation of graphite. The chemical and mechanical exfoliation methods are suitable for the large-scale production of graphene flakes. The quality of the graphene produced is very low based on the electronic, thermal, strength, lateral dimensions, etc. The micromechanical exfoliated method used to isolate graphene for the first time in 2004 can also be classified in this top down approach. However, unlike the other two bulk production methods, this method leads to extremely small quantities of high quality micrometer scale monolayer graphene flakes. The main disadvantage of this technique is that it cannot be scalable to sizes that are large enough to be useful for industrial applications.
In the case of the bottom up approach the graphene is formed via the rearrangements of the carbon atoms in a Chemical Vapor Deposition (CVD) process. The sublimation of silicon from silicon carbide substrates can also be classified into this group. The only problem with this fabrication method is that the maximum graphene size that can be manufactured is limited by the substrate size that at present is at four-inch wafer scale. In addition the silicon carbide substrates are extremely expensive and would make graphene exceedingly expensive.
Large area graphene films have been manufactured using CVD methods. In 2010 a thirty-inch graphene film was manufactured for touch screen applications and published by Sukang Bae, et al, “Roll-to-roll production of 30-inch graphene films for transparent electrodes” in Nature Nanotechnology vol. 5, pg. 574-579, 2010.
The quality in terms of properties of the graphene produced via CVD is far superior to the graphene flakes produced using the bulk production methods.
In the CVD process graphene is deposited at relatively high temperatures between 600 and up to 1000° C. on a metal catalyst such as copper and nickel. Copper has been reported to control much better the deposition of monolayer graphene in comparison to nickel. The first publication of graphene growth on copper was reported in Science in 2009 by Xuesong Li et al, “Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils” Science vol. 324, pg. 1312-1314, 2009 and in US patent US20110091647A1.
On the other hand graphene growth in nickel was reported at the beginning of 2009 in Nano Letters by Alfonso Reina et al, “Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical Vapor Deposition” in Nano Letters vol. 9, pg. 30-35, 2009 and in US patent US20100021708A1 where multilayer regions are quite abundant. During the CVD process the metal catalyst is exposed to a carbon source (solid, liquid or gas) at relatively high temperatures in order to deposit the graphene. Graphene deposition can be done close to atmospheric pressure conditions or under vacuum. The mechanism of the graphene formation can vary depending on the metal catalyst type. In the case of nickel catalysts graphene is primarily formed during the cooling down stage as a consequence of a precipitation process. The formation of graphene on copper occurs at elevated temperatures and it can be self-limiting up to some extend. As a result this surface reaction stops when the copper surface has been completely covered. A monolayer graphene coverage higher than 95% can be obtained on top of copper.
The copper catalyst can be in the form of thin films on top of silicon substrates or thicker films in the form of foils. In the case of the copper foils the graphene growth occurs at either side of the foil. As a consequence the bottom graphene layer has to be removed if monolayer graphene on insulating substrates is the required product.
The removal of one of the graphene layers has been reported using oxygen plasma etching in WO2012031238A2 and WO2012021677A2. This is the most common and reported method to eliminate the bottom graphene layer. However, the plasma etching method has a number of limitations:                Can end up damaging the top monolayer graphene layer (end product)        Not easily transferrable due to specific equipment dependency        Usually involves costly equipment        Not easy for inline integration, as a consequence could become the process bottleneck        Vacuum conditions required        
Therefore alternative methods are highly desired in order to overcome these shortcomings and enable large-scale manufacturing and transfer of graphene films to be applied in potential industrial applications.
WO2012031238A2 shows a transport step where the graphene is transferred from the copper foil onto silicon substrates using adhesive polymers. The impact on the uniformity of the final monolayer graphene produced using this transfer step is not presented. However, this transfer step can have a detrimental effect on the homogeneity, uniformity and in turn quality of the desired graphene layer. For the person skilled in the art this conclusion is pretty apparent especially in the case of large area graphene films. We must point out, however, that it is possible to satisfactorily transfer small areas (micrometer scale) of monolayer graphene using this technique. Similarly to the way graphene was initially discovered where a Scotch tape was used to transport the graphene from the starting graphite up to the final insulating substrate. Micrometer scale flakes were transferred without jeopardizing the homogeneity, uniformity and quality of the graphene flake.
In the same line of thought, if stamping techniques are used to transport graphene onto insulating substrates they could potentially damage it. Polydimethylsiloxane (PDMS) stamps have been used only to transfer relatively small areas of monolayer graphene as published in WO20122021677A2. However for a person skilled in the art of graphene transfer processes it is quite evident that this method can have scale-up issues in order to transfer large graphene films. In other words, if the complete transfer of small films is not possible then the full transfer of larger films would be less likely.